Rheumatology and Immunology
Core concepts
Understand the pattern-recognition approach to arthritis: monoarticular (septic, crystal, trauma), oligoarticular, and polyarticular (RA, SLE, viral). Know the serologies - rheumatoid factor and anti-CCP for RA, ANA for SLE with anti-dsDNA and anti-Smith as specific markers, and ANCA patterns (c-ANCA/PR3 for GPA, p-ANCA/MPO for MPA and EGPA). Cover the vasculitides, the spondyloarthropathies (HLA-B27), and the complement levels (low in active SLE). Learn the complications of immunosuppressive therapy and the approach to a patient on biologics with infection.
Traps
Septic arthritis must be excluded before assuming a flare - joint aspiration is mandatory for a hot swollen joint. Exam questions test the difference between RA (symmetric, small joints, morning stiffness >1 hour, erosions) and osteoarthritis (weight-bearing joints, brief stiffness, osteophytes). Remember drug-induced lupus (hydralazine, procainamide, isoniazid) with anti-histone antibodies, and that ANA positivity is common in healthy people - specificity matters.
Worked rule of thumb
Hot, swollen, single joint: aspirate before treating. Cell count >50,000/mm³ with neutrophilia and positive Gram stain suggests septic arthritis; needle-shaped negatively birefringent crystals indicate gout, and rhomboid positively birefringent crystals indicate pseudogout.
Key points
- Aspirate any hot monoarthritis to exclude sepsis.
- Anti-CCP is more specific than rheumatoid factor for RA.
- Anti-dsDNA and anti-Smith are specific for SLE.
- c-ANCA/PR3 associates with granulomatosis with polyangiitis.
- Drug-induced lupus is associated with anti-histone antibodies.